Iontophoresis, according to Dorland's Illustrated Medical Dictionary, is defined to be "the introduction, by means of electric current, of ions of soluble salts into the tissues of the body for therapeutic purposes." Iontophoretic devices have been known since the early 1900's. British patent specification No. 410,009 (1934) describes an iontophoretic device which overcame one of the disadvantages of such early devices known to the art at that time, namely the requirement of a special low tension (low voltage) source of current which meant that the patient needed to be immobilized near such source. The device of that British specification was made by forming a galvanic cell from the electrodes and the material containing the medicament or drug to be delivered transdermally. The galvanic cell produced the current necessary for iontophoretically delivering the medicament. This ambulatory device thus permitted iontophoretic drug delivery with substantially less interference with the patient's daily activities.
More recently, a number of United States patents have issued in the iontophoresis field, indicating a renewed interest in this mode of drug delivery. For example, U.S. Pat. No. 3,991,755 issued to Vernon et al; U.S. Pat. No. 4,141,359 issued to Jacobsen et al; U.S. Pat. No. 4,398,545 issued to Wilson; and U.S. Pat. No. 4,250,878 issued to Jacobsen disclose examples of iontophoretic devices and some applications thereof. The iontophoresis process has been found to be useful in the transdermal administration of medicaments or drugs including lidocaine hydrochloride, hydrocortisone, fluoride, penicillin, dexamethasone sodium phosphate, insulin and many other drugs. Perhaps the most common use of iontophoresis is in diagnosing cystic fibrosis by delivering pilocarpine salts iontophoretically. The pilocarpine stimulates sweat production; the sweat is collected and analyzed for its chloride content to detect the presence of the disease.
In presently known iontophoretic devices, at least two electrodes are used. Both of these electrodes are disposed so as to be in intimate electrical contact with some portion of the skin of the body. One electrode, called the active or donor electrode, is the electrode from which the ionic substance, medicament, drug precursor or drug is delivered into the body by iontophoresis. The other electrode, called the counter or return electrode, serves to close the electrical circuit through the body. In conjunction with the patient's skin contacted by the electrodes, the circuit is completed by connection of the electrodes to a source of electrical energy, e.g., a battery. For example, if the ionic substance to be delivered into the body is positively charged (i.e., a cation), then the anode will be the active electrode and the cathode will serve to complete the circuit. If the ionic substance to be delivered is negatively charged (i.e., an anion), then the cathode will be the active electrode and the anode will be the counter electrode.
Alternatively, both the anode and cathode may be used to deliver drugs of opposite charge into the body. In such a case, both electrodes are considered to be active or donor electrodes. For example, the anode can deliver a positively charged ionic substance into the body while the cathode can deliver a negatively charged ionic substance into the body.
It is also known that iontophoretic delivery devices can be used to deliver an uncharged drug or agent into the body. This is accomplished by a process called electroosmosis. Electroosmosis is the transdermal flux of a liquid solvent (e.g., the liquid solvent containing the uncharged drug or agent) which is induced by the presence of an electric field imposed across the skin by the donor electrode.
Furthermore, existing iontophoresis devices generally require a reservoir or source of the beneficial agent (which is preferably an ionized or ionizable agent or a precursor of such agent) to be iontophoretically delivered into the body. Examples of such reservoirs or sources of ionized or ionizable agents include a pouch as described in the previously mentioned Jacobsen U.S. Pat. No. 4,250,878, or a pre-formed gel body as described in Webster U.S. Pat. No. 4,383,529 and Ariura et al. U.S. Pat. No. 4,474,570. Such drug reservoirs are electrically connected to the anode or the cathode of an iontophoresis device to provide a fixed or renewable source of one or more desired agents.
More recently, iontophoretic delivery devices have been developed in which the donor and counter electrode assemblies have a "multi-laminate" construction. In these devices, the donor and counter electrode assemblies are each formed by multiple layers of (usually) polymeric matrices. For example, Parsi U.S. Pat. No. 4,731,049 discloses a donor electrode assembly having hydrophilic polymer based electrolyte reservoir and drug reservoir layers, a skin-contacting hydrogel layer, and optionally one or more semipermeable membrane layers. Sibalis U.S. Pat. No. 4,640,689 discloses in FIG. 6 an iontophoretic delivery device having a donor electrode assembly comprised of a donor electrode (204), a first drug reservoir (202), a semipermeable membrane layer (200), a second drug reservoir (206), and a microporous skin-contacting membrane (22'). The electrode can be formed of a carbonized plastic, metal foil or other conductive films such as a metalized mylar film. In addition, Ariura et al, U.S. Pat. No. 4,474,570 discloses a device wherein the electrode assemblies include a conductive resin film electrode layer, a hydrophilic gel reservoir layer, a current distribution and conducting layer and an insulating backing layer. Ariura et al disclose several different types of electrode layers including an aluminum foil electrode, a carbon fiber non-woven fabric electrode and a carbon-containing rubber film electrode.
Others have suggested using biomedical electrodes having current distribution members composed of a rubber or other polymer matrix loaded with a conductive filler such as powdered metal. See for example, U.S. Pat. No. 4,367,745. Such films however, have several disadvantages. First, as metal particle loading in a polymer matrix approaches about 65 vol%, the matrix begins to break down and becomes too brittle to be handled. Even at metal particle loadings only about 50 to 60 vol%, the films produced are extremely rigid and do not conform well to non-planar surfaces. This is a particular disadvantage when designing an electrode adapted to be worn on the skin or a mucosal membrane. An iontophoretic electrode adapted to be worn on a body surface must have sufficient flexibility to contour itself to the natural shape of the body surface to which it is applied.
The drug and electrolyte reservoir layers of iontophoretic delivery devices have been formed of hydrophilic polymers. See for example, Ariura et al, U.S. Pat. No. 4,474,570; Webster U.S. Pat. No. 4,383,529 and Sasaki U.S. Pat. No. 4,764,164. There are several reasons for using hydrophilic polymers. First, water is the preferred solvent for ionizing many drug salts. Secondly, hydrophilic polymer components (i.e., the drug reservoir in the donor electrode and the electrolyte reservoir in the counter electrode) can be hydrated while attached to the body by absorbing water from the skin (i.e., through transepidermal water loss or sweat) or from a mucosal membrane (e.g., by absorbing saliva in the case of oral mucosal membranes). Once hydrated, the device begins to deliver ionized agent to the body. This enables the drug reservoir to be manufactured in a dry state, giving the device a longer shelf life.
The prior art has also recognized that certain electrode compositions are preferred from the standpoint of drug delivery efficiency and minimizing skin burns caused by pH extremes. For example, U.S. Pat. Nos. 4,744,787; 4,747,819 and 4,752,285 all disclose iontophoretic electrodes which are either oxidized or reduced during operation of the device. Preferred electrode materials include a silver anodic electrode, which is used to deliver the chloride salt of a drug, and a silver/silver chloride cathodic (return) electrode. Silver ions generated at the anode combine with the drug counter ion (i.e., chloride ions) to produce an insoluble silver chloride precipitate. This reduces competition between the drug ions and the silver ions for delivery into the body and increases the efficiency of the device.